It is known that the mechanical properties of plastic pipe can be improved by incorporation of discontinuous reinforcing fibers in it. It is also known that properties of the fiber reinforced plastic tube are strongly dependent on fiber orientation. A significant improvement in certain properties of fiber reinforced plastic tube occurs in the direction of preferential fiber orientation.
There have been a number of methods and apparatus proposed for making plastic tubes. Known in the art is a process for controlling orientation of discontinuous fiber in fiber reinforced tube formed by extrusion (U.S. Pat. No. 4,056,591, November 1, 1977 Goettler). The process described in this patent comprises extruding a mixture of fibers and matrix material through a diverging channel of essentially constant width formed by stationary die members. The outlet area of the die channel being at least two or more times the channel inlet areas, this relation between outlet and inlet area provides plastic tube having improved physical properties in the hoop direction.
Another known method of extruding fiber reinforced plastic tube (U.S. Pat. No. 3,933,960 January 20, 1976 Cameron et al) comprises continuously extruding at least one viscous material having reinforced fibers therein through two concentric sets of converging discrete passages, producing laminar flow in the passages by causing the material to accelerate on entry into the passages and preventing deceleration thereof within the passages whereby the fibers orientate themselves in the material in a lengthwise direction of the passages, and then bringing together the extruded material from the respective passages so as to form two layers of material one within the other with the fibers of each layer lying on helices of opposite hands with respect to one another by passage through rotating die members.
Although all the above processes are suitable for providing plastic tubes with reinforced fibers, none of these processes can produce in a satisfactory manner multi-layered plastic tubes having an intermediate layer containing discrete fibers where a major portion of the fibers is oriented to extend predominantly in a general direction lying between the circumferential and the longitudinal axis of the tube with the orientation of the fibers being generally the same throughout the thickness of the intermediate layer. Theoretical and experimental research have shown that while a die similar to the one taught by Goettler can give rise to a preferential circumferential orientation of the fibers in a certain region of material, a significant longitudinal orientation is unavoidable in regions near the walls of the die. The reason is that the flow through such a die involves a superposition of shear and extensional components of deformation. While the shear component tends to orient the fibers in the longitudinal direction, the extensional component tends to orient them in the circumferential direction. The magnitude of the extensional component becomes insignificant near the wall of the die as the shear component approaches a maximum, therefore the fibers tend to a longitudinal orientation near the inner and outer walls of the die. This method does not provide fibers having the same orientation throughout the thickness of the tube. The process described by Cameron et al provides a multi-layered tube with helically oriented fibers. The rotating die members taught by Cameron et al limits the process to providing reinforced fibers with helical orientation as distinct from circumferential and longitudinal orientation.
There is a need for a method of, and an apparatus for, making discrete fiber reinforced plastic tube having an improved overall fracture toughness.
There is also a need for a method of, and an apparatus for, making discrete fiber reinforced plastic tube having an improved stiffness and strength.
There is also a need for a method of, and an apparatus for, making discrete fiber reinforced plastic tube having an intermediate layer of plastic material containing discrete fibers, where a major portion of the fibers is oriented to extend predominantly in a general direction lying between the circumferential and the longitudinal axis of the tube with the orientation of the fibers being generally the same throughout the thickness of the intermediate layer, by a simple and continuous process.